Defending champion Jack Sock advances in Houston

Jack Sock of the United States returns a shot to Matthew Barton of Australia at the U.S. Men's Clay Court Championship tennis tournament Thursday, April 7, 2016, in Houston. Sock won 6-2, 7-6.(AP Photo/Pat Sullivan)
(The Associated Press)

Jack Sock of the United States returns a shot to Matthew Barton of Australia at the U.S. Men's Clay Court Championship tennis tournament Thursday, April 7, 2016, in Houston. Sock won 6-2, 7-6.(AP Photo/Pat Sullivan)
(The Associated Press)

Jack Sock of the United States returns a serve to Matthew Barton of Australia at the U.S. Men's Clay Court Championship tennis tournament Thursday, April 7, 2016, in Houston. Sock won 6-2, 7-6.(AP Photo/Pat Sullivan)
(The Associated Press)

The fourth-seeded American won last year at River Oaks for his first ATP Tour title. He will face the winner of a match between seventh-seeded Marcos Baghdatis of Cyprus and Fernando Verdasco of Spain.

Fellow Americans John Isner and Sam Querrey reached the quarterfinals Wednesday. The top-seeded Isner will face South Korea's Chung Hyeon, and the fifth-seeded Sam Querrey will play Argentina's Juan Monaco.